Birds of a Feather

Why do chickens molt when the colder weather starts? I have one that started last week, which isn't bad. By the time it freezes, she'll be refeathered. But there are always a few who are still rather threadbare when we're getting some frosts, or even downright freezes, at night. I knew before getting chickens that they molt each year, but the first year it happened I thought one of them had exploded! LOL There are a LOT of feathers on those girls. I haven't actually seen any of them losing any (our house is about 100 ft. from their pen), but it looks like they shake and fluff their feathers and lose a bunch in that spot. Then later, when they're in a different spot, they shake again and lose more. Some of them get really pathetic looking and with others, you'd never know they were molting except when they spread their wings and some flight feathers are missing. Here are some of the girls.